Incorporate a testing mode for the Staging pipeline.

[epananth]

Create a test environment for the staging pipeline separating testing and production resources. Currently the test staging pipeline publishes to temporary containers in dotnetstage and dotnetcli storage accounts mixing test and production assets

Goals

• Create a test environment ideally in a separate subscription:
  • Storage accounts: testing counterparts of dotnetstage, dotnetchecksumsstage, dotnetcli, dotnetclichecksums
  • Key vaults containing the secrets for reading and writing to those storage accounts: test counterparts of dotnetstagekeys, dotnetstagetokens, dotnetclikeys, dotnetclitokens
• Create test and production variable templates containing the corresponding secrets and locations for publishing
• Include the correct template (test/production) in all the stages
• Remove the creation of temporary containers and use the testing ones
• Improve testing for stages which are currently skipped: Retain build drop, copying to file shares

Proof of concept: https://github.com/dotnet/release/issues/689